The conventional wisdom about Arsenal's title push is that they have the easy run-in. Compared to Chelsea and Manchester United it is apparently a piece of cake. A home game against the team above the relegation zone by virtue of goal difference was supposed to be one of the tastiest of the lot, but Arsène Wenger's team came oh-so-close to suffering a terrible dose of indigestion.

The game hinged on a critical incident a minute before half-time, when Thomas Vermaelen was shown a red card for tangling with Guillermo Franco. Not so easy all of a sudden. But the way Arsenal regrouped, resettled, and finished off West Ham showed they have the heart to take this adventure as far as they possibly can. Their title rivals may have games in hand, but Arsenal wake up this morning looking down on everyone.

The quest now moves to Birmingham. For different reasons recent games have all felt central to the plot of this unfolding drama, but St Andrew's is a challenge that feels particularly pivotal in terms of Premier League aspirations. Against the kind of direct opponents they have been known to find unsettling, they will travel without their first-choice centre-halves. As a prelude to Barcelona, its importance cannot be overlooked.

As much as it was natural for Arsenal to be thankful for the return of their captain and top scorer Cesc Fábregas, the reinstatement of Alex Song after a two-match suspension was equally reassuring. The Cameroonian anchor has been crucial, and has matured into an important safety net in front of a back line with a tendency towards fragility.

Alongside him Denílson was chosen ahead of Abou Diaby. This was surprising. A more creative player is the norm alongside Fábregas and Song in midfield, but Denílson was favoured ahead of Diaby and Tomas Rosicky. Was this an experiment with Barcelona in mind?

Arsène Wenger was firm that West Ham was the absolute priority, and the Denílson selection turned out to be an inspired one. The Brazilian provided his team with the gift of an early goal. He was alert to the opportunity to pickpocket Valon Behrami and skillfully smuggled the ball to Nicklas Bendtner. When it was returned to him, Denílson drilled a first-time shot into the bottom corner.

Denílson has elicited his fair share of moans from the crowd this season, especially during a period when he found the going tough in midwinter. But here he sparked. Maybe he had borrowed some of Bendtner's enormous supply of confidence (there is plenty to spare). This was his fourth league goal from 16 starts this season, and not for the first time it was an important goal, too.

He might have had another soon after. At the end of a tippy-tappy move, Denílson chested the ball down and volleyed goalwards. His flourishes were all the more valuable as Arsenal were not at their fluent best in the first half. Fábregas took an early kick on the foot and looked very unhappy with the perpetrator, Behrami, with whom he later had words. The Catalan was not running freely at all and strained to exert any great influence. Samir Nasri and Andrey Arshavin were a little flat, too.

West Ham had enough glimmers to suggest Arsenal would be foolish to take this at too much of a presumptuous stroll. Junior Stanislas broke down the right flank and whipped in an inviting cross which Mido couldn't reach, then Gaël Clichy and Sol Campbell made excellent interceptions as West Ham built towards goal.

In the last minute of the half, the pendulum swung viciously. Franco surged onto a high pass and Vermaelen missed the header, then in his desperation to retrieve the situation was clumsy as he tussled with the Mexican. Although contact was minimal, Franco tumbled inside the penalty area. The referee Martin Atkinson was so far behind the play he was closer to the centre circle than the penalty box, but trusted the instincts of his linesman. Not only did he point to the spot, he sent off Vermaelen. Wenger was infuriated, and waited at half-time to remonstrate with the officials.

In the meantime, Diamanti struck his kick well, but Manuel Almunia plunged to produce an inspired save. His record with penalties is one of his best features, and Arsenal were immensely grateful.

Interestingly, Wenger chose not to make a substitution and Song dropped back to fill in at centre-half. He had his work cut out as West Ham set about the second half with attack in their hearts. It was not long before Arsenal did make a change, with Diaby replacing Bendtner and Arshavin leading the line.

Arsenal hauled themselves back again to force the game up towards the edge of Rob Green's box. Emmanuel Eboué became increasingly influential, and his ability to win free-kicks kept up the pressure. Campbell ambled up for a corner but headed too close to the keeper.

Gianfranco Zola, so desperate for points, sent on the attacking power of Carlton Cole and experience of Benni McCarthy. With 12 minutes to go a sizzling left-footer from Cole shuddered against the base of Almunia's far post.

Back came Arsenal, and Matthew Upson handled in the box inexplicably as Fábregas bore into the danger zone. Another penalty. In the swirling rain, the captain steeped up to rifle in, Green diving the wrong way. "We are top of the league," sang the crowd giddily. It ain't easy, but it sure is scintillating.

THE FANS' PLAYER RATINGS AND VERDICT

LOUISE COWBURN, Observer reader I'm absolutely delighted. It felt so good when we were singing "We are top of the league" – even if it is only for half a day. It's been absolutely ages since we had that kind of atmosphere in the stadium and it showed how resilient we are as a team to play for half a game when we were down to 10 men. That totally dispelled the idea that we are all style above substance. We really ground it out. I didn't see if Vermaelen should have been sent off but from the crowd reaction it seemed harsh. But he walked off like a true gent with no complaining. After he went, Song was absolutely brilliant when he dropped back.

TIM & TOM CONLAN, Observer readers Two words: utter disgrace. I didn't like the selection – why Cole was on the bench was beyond me. It shows the negative attitude of the entire club at the moment. There is no ambition: when you're in a relegation fight you have to throw caution to the wind. Arsenal weren't that good, especially after they were down to 10 men. Our midfield battled but there were no proper balls, no quality round the box. Forget the penalty – even if we had scored I still think Arsenal would have come back to win 3-1. It wasn't even a penalty to begin with. They were down to 10 men and they were still the better side.